If your dog is not neutered then it could be there's a female dog in heat somewhere and his hormone change has taken place as at that time a dog's urine changes to a strong odor. (Is he marking drapes, furniture, etc?)

Also, it could be your dog is having a health issue and needs antibiotics to clear up an infection. I would take him to the vet just to be safe.

Is your dog getting enough water? Sometimes dark, smelly urine is because they are dehydrated and not getting enough fresh water. Their urine concentrates and appears to be very dark yellow and even thick looking. Take him to your vet. You might try changing his normal food and get him on a senior light diet. Is he overweight? He could have an infection or the beginnings of renal failure. Or perhaps some type of obstruction. Good luck. I wish him WELL. He is gorgeous!

Before spending money on the vet, cut his food intake in half and give him plenty of fresh water. Try two bowls - one cold water and one water with ice cubes. If you don't see a noticeable improvement, then go to the vet.